A variety of image processing is executed in the prior art by obtaining the feature values of an original image and then utilizing these feature values to execute the processing.
One example of a method of obtaining such a feature value from an original image involves dividing the original image (the input image) into blocks of vertical and horizontal numbers of pixels that are to undergo scaling, and calculating the mean values of the pixels (pixel values) within a plurality of the block images produced by the aforementioned division, thereby generating a scaled image of the original image. The scaled image thus produced is then subjected to the well-known discrete cosine transform (“DCT” below) and quantization processing, as a result of which coefficients are obtained. Several coefficients from the side of low-frequency components are extracted from these coefficients as a feature of the original image. The feature thus extracted can be employed as data used in an image search. [See ISO/IEC JTC1/SC29/WG11/N3522 “MPEG-7 Visual Working Draft 4.0” (VWD 4.0) or ISO/IEC JTC1/SC29/WG11/N3522 “MPEG-7 Visual part of experimentation Model Version 7.0” (VXM 7.0).]
The conventional procedure for extracting feature values will be described. FIG. 1 is a diagram useful in describing the flow of processing for extracting a color layout descriptor value. This procedure is described in VWD 4.0 or VXM 7.0. FIG. 3 is a flowchart illustrating the processing for extracting a color-layout descriptor value.
In FIGS. 1 and 3, an original image 10001 is scaled to a plurality of blocks of 8×8 pixels each (step S10201). When the scaled image of the original image is produced, use is made of the mean value of the pixels within each block obtained by dividing the original image into vertical and horizontal numbers of pixels that are the target of scaling.
The pixels constituting the generated block images (10011, 10012, 10013) of 8×8 pixels each are converted to data (10021, 10022, 10023) in Y, Cb, Cr color space (step S10202).
Next, the data 10021, 10022, 10023 representing the components in Y, Cb, Cr color space is subjected to DCT processing (step S10203), whereby DCT coefficients 10031, 10032, 10033 are acquired, and the DCT coefficients 10031, 10032, 10033 are then subjected to quantization (step S10204).
In accordance with VWD 4.0 or VXM 7.0, the above-mentioned image scaling processing, color conversion processing and DCT processing may be implemented through well-known techniques and the processing is not particularly standardized.
Further, with regard to image scaling processing, VWD 4.0 or VXM 7.0 merely recommend dividing the original image into blocks of 8×8 pixels each and adopting the average color of the pixels within each block. For example, in accordance with VWD 4.0, quantization processing for DC components differs from that for AC components with regard to the Y component and Cb/Cr components.
Next, several coefficients are selected from the side of low-frequency components among the quantized DCT coefficients (10041, 10042, 10043) obtained as a result of quantization processing (step S10205). In the example of FIG. 1, six coefficients (10051) have been selected with regard to the coefficients of the Y component, and three coefficients each (10052, 10053) have been selected with regard to the coefficients of the Cb/Cr components.
FIG. 2 is a diagram useful in describing zigzag-scan processing for selecting coefficients. As exemplified in FIG. 2, the selection of coefficients at step S10205 is achieved by rearranging the coefficients, which are arrayed two-dimensionally as indicated by the 8×8 pixel configuration, into a one-dimensional array by zigzag scanning, and selecting several coefficients starting from the leading coefficient. The numerals 1 to 64 written in the blocks of FIG. 2 indicate which numbers the coefficients will come to occupy starting from the leading coefficient after the coefficients have been rearranged one-dimensionally.
In accordance with VWD 4.0, the coefficients that should be selected in the coefficient selection process are any of 1, 3, 6, 10, 15, 21, 28 and 64. Though the numbers of coefficients are the same for the Cb-component coefficients and Cr-component coefficients, it is possible for the number of Y-component coefficients to be set to a number different from that of the Cb/Cr-component coefficients. With VWD 4.0, the default selection is six coefficients with regard to the Y-component coefficients and three coefficients for each of the Cb/Cr-component coefficients. In other words, VWD 4.0 adopts feature data (a color layout descriptor value) representing the color layout of the original image 1001 using the selected coefficients 10051, 10052, 10053.
If use is made of color layout descriptor values calculated as set forth above with regard to a plurality of images, similar images can be retrieved. The degree of similarity between items of feature data is calculated as follows in accordance with VXM 7.0. For example, degree of similarity D between a color layout descriptor value CLD1 (YCoeff, CbCoeff, CrCoeff) and a color layout descriptor value CLD2 (YCoeff′, CbCoeff′, CrCoeff′) of two images is calculated in accordance with the following equation:
                    D        =                                                            ∑                                  i                  =                  0                                                                      Max                    ⁢                                          {                                              Number                        ⁢                                                                                                  ⁢                        Of                        ⁢                                                                                                  ⁢                        YCoeff                                            }                                                        -                  1                                            ⁢                                                                    λ                    Yi                                    ⁡                                      (                                                                  YCoeff                        ⁡                                                  [                          i                          ]                                                                    -                                                                        YCoeff                          ′                                                ⁡                                                  [                          i                          ]                                                                                      )                                                  2                                              +                                                    ∑                                  i                  =                  0                                                                      Max                    ⁢                                          {                                              Number                        ⁢                                                                                                  ⁢                        Of                        ⁢                                                                                                  ⁢                        YCCoeff                                            }                                                        -                  1                                            ⁢                                                                    λ                    Cbi                                    ⁡                                      (                                                                  Cboeff                        ⁡                                                  [                          i                          ]                                                                    -                                                                        YCboeff                          ′                                                ⁡                                                  [                          i                          ]                                                                                      )                                                  2                                              +                                                    ∑                                  i                  =                  0                                                                      Max                    ⁢                                          {                                              Number                        ⁢                                                                                                  ⁢                        Of                        ⁢                                                                                                  ⁢                        CCoeff                                            }                                                        -                  1                                            ⁢                                                                    λ                    Cri                                    ⁡                                      (                                                                  CrCoeff                        ⁡                                                  [                          i                          ]                                                                    -                                                                        CrCoeff                          ′                                                ⁡                                                  [                          i                          ]                                                                                      )                                                  2                                                                        (        1        )            
In the above equation, λ indicates weighting relating to each coefficient. Weighting values of the kind shown in Table 1 below are indicated in VXM 7.0. The cells in Table 1 that do not show a value have weighting values of 1.
TABLE 1ORDER OF COEFFICIENTS123456Y222111Cb211Cr422
When a scaled image of an original image is produced at step S10201 in the above-described conventional method of extracting feature values, use is made of the mean value of the pixels within a block obtained by dividing the original image into vertical and horizontal numbers of pixels that are the target of scaling. As a consequence, information relating to the composition of the original image is unclear.
Furthermore, several coefficients are extracted from the low-frequency component side of quantized coefficients obtained by applying DCT processing to a scaled image, as described above. As a consequence, the smoothing effect on the original image as a total system becomes too extreme, thereby making the composition information of the original image even more obscure.